Usually, a copper foil which is used for a printed circuit board substrate which forms the base of a printed circuit board, multilayer printed circuit board, chip-on-film-use circuit board, etc. is roughened at its surface on the side to be hot bonded to a resin substrate or the like. This roughened surface exhibits an anchoring effect for the substrate. This thereby raises the bond strength between the substrate and the copper foil to secure reliability as the printed circuit board substrate.
In recent years, in circuit boards, to deal with the high integration of various types of electronic parts, finer line widths and line spacing for interconnect patterns have been increasingly requested. For example, in a case of a printed circuit board used for a semiconductor package, it has been requested to provide a printed circuit board having high density ultra-fine interconnects with line widths and line spacing of approximately 30 μm, respectively.
When using a thick copper foil as such a copper foil for a fine pattern printed circuit board, an etching time to form interconnect circuits by etching becomes longer. As a result, the verticality of the side walls of the interconnect patterns formed is ruined. When the interconnect line widths of the interconnect patterns formed are narrow, this sometimes leads to disconnects. Accordingly, as the copper foil used for fine pattern applications, a copper foil having a thickness of 9 μm or less is demanded. At present, a copper foil having a thickness of 5 μm or less is being frequently used.
However, such 9 μm or less thin copper foil (hereinafter, sometimes referred to as an “ultra-thin copper foil”) is weak in mechanical strength, easily suffers from wrinkles or creases at the time of production of the printed circuit board substrate, and sometimes causes tearing of the copper foil. Therefore, as the ultra-thin copper foil used for fine pattern applications, an ultra-thin copper foil with a carrier obtained by directly electrodepositing an ultra-thin copper foil layer on one surface of a carrier comprised of a metal foil (hereinafter, referred to as a “carrier foil”) through a release layer is being used. The copper foil of a thickness of 5 μm or less used at present and explained above is provided as this ultra-thin copper foil with a carrier.
An ultra-thin copper foil with a carrier is comprised of a carrier foil on one surface of which a release layer and an ultra-thin copper foil formed by electroplating of copper are formed in that order. The surface of the outermost layer of the ultra-thin copper foil formed by the electroplating of copper is finished to a roughened surface. Then, the roughened surface is superposed on a resin base material, then the entire assembly is hot bonded, then the carrier foil is peeled off to thereby to form a copper-clad laminate board. Predetermined interconnect patterns are formed on the ultra-thin copper foil at the surface of the copper-clad laminate board in this mode of use.
When peeling off of the carrier foil after hot bonding the ultra-thin copper foil to the resin base material, the problem arises of deformation of the ultra-thin copper foil if the ultra-thin copper foil has a thickness less than 5 μm. Therefore, it has been considered necessary to make a peeling strength when peeling off the carrier foil and the ultra-thin copper foil (hereinafter, sometimes referred to as a “carrier peel”) low and stable.
A carrier foil functions as a reinforcing material for stably maintaining the shape of the ultra-thin copper foil until bonding the ultra-thin copper foil to the resin base material. The release layer is a layer for improving the peeling when separating the ultra-thin copper foil from the carrier foil. By removal together with the carrier foil, the carrier foil can be cleanly and easily peeled off.
The copper-clad laminate board comprised of the resin base material with the ultra-thin copper foil bonded to it is then successively formed with through holes and plating at the through holes. Then, the ultra-thin copper foil at the surface of the copper-clad laminate board is etched to form interconnect patterns provided with desired line widths and desired line spacing. Finally, a solder resist is formed and other finishing work is performed.
For the release layer formed on the carrier foil, conventionally use has been made of benzotriazole or another organic system, a metal oxide or another inorganic system, a solo metal element, alloy, or other metal system, and so on. However, in a circuit board substrate using a polyimide or other highly heat resistant resin as the resin base material, a heating temperatures when pressing the copper foil and a resin and curing the resin becomes high. This cannot be handled by an organic-based release layer where peeling becomes difficult due to a rise of the peeling strength caused by this heat treatment, so a metal-based release layer is mainly being used.
As the metal of forming a metal-based release layer, Cr (PLT 1), Ni (PLTs 1 and 2), Ti (PLT 3), and so on have been proposed up to now.
These proposals have as characteristic features rendering part of the metal forming the release layer into an oxide state adjusting the carrier peel according to a ratio of content between the metal and the oxides in the release layer and the thickness of the oxide coating film formed at the surface of the metal. However, due to differences of the ratio of the oxide content and thickness of the oxide coating film, there were the problems that variations easily arose in the carrier peel, the carrier peel was hard to stabilize, and blistering occurred.
Further, at the surface of oxides, precipitation by electroplating of copper is hard to occur, so it is difficult to uniformly plate the release layer. Therefore, sometimes there also arise the problems of variation of thickness of the ultra-thin copper foil and occurrence of pinholes.
Contrary to this, it has been proposed to continuously change an oxygen concentration inside a release layer in a release layer made using a plurality of metals and oxides of the same so as not to form a clear interface and thereby to prevent blistering which occurs due to a difference of thermal expansion coefficient etc. at an interface (PLT 4). However, since the interface at which peeling is liable to occur does not become constant, the problem of the carrier peel becoming higher and harder to stabilize could not be solved.
FIG. 3 schematically shows a carrier peel of a conventional copper foil with a carrier, in which 1 indicates an ultra-thin copper foil, 2 a carrier foil, 8 a release layer, 9 the position where blistering occurred, and dotted lines indicate portions where the peeling strength is weak. FIG. 3A shows a state where blistering occurred due to existence of a portion having a weak adhesion between the ultra-thin copper foil and the release layer, FIG. 3B shows a state where the carrier peel has variation, there are parts where peeling is difficult, and the carrier peel becomes high, while FIG. 3C shows a state where the carrier peel has variation, so the carrier peel does not become stable.
As one means for solving these problems, the present applicant previously developed an ultra-thin copper foil with a carrier suppressing occurrence of blistering, exerting no influence upon a carrier peel, stable in production quality, and easily capable of peeling apart the carrier foil and electrolytic copper foil even under a high temperature environment (PLTs 5 and 6).